Lithostratigraphy and integrated diatom and calcareous nannofossil biostratigraphy of the Miocene sequence in the Iwadono Hills area, Saitama Prefecture, central Japan

Lithostratigraphy is revised for the Miocene marine sequence in the Iwadono Hills area. Saitama Prefecture, central Japan, and integrated diatom and calcareous nannofossil biostratigraphy is established for the lower part of the sequence. The Miocene sequence in this area is subdivided into the Kamikarako, Godo, Negishi(redefined), Shogunzawa(redefined). Hatoyama, and Imajuku Formations in ascending order. These formations are essentially conformable except for the boundary between the Kamikarako and Godo Formations, where the Godo Formation is considered to overlie the Kamikarako Formation unconformably. Important nine key tuff beds for the local correlation of the Negishi and Shogunzawa Formations are described. The main part of the Kamikarako Formation is assigned to the Denticulopsis lauta Zone (NPD 4A) within the framework of North Pacific diatom zonation, but the lower most part of the formation is correlative with the Crucienticula Kanayae Zone (NPD 3A). The Negishi and the lowermost Shogunzawa Formations are assigned to the basal part of the Denticulopsis praedimorpha Zone (NPD 5B). Calcareous nannofossil biostratigraphic analysis indicates that the lower oart of the Negishi Formation is assignable to the Zone CN 4 and that the upper Negishi and the lowermost Shogunzawa Formations are correlative with the Subzone CN 5a. On the basis of the microfossil biochronology, geologic age of the unconformity between the Kamikarako and Godo Formations is restricted to 15.1-15.4 Ma of Middle Miocene. This unconformity is correlatable to the widespread uncoformity recognized in a number of Miocene sequences in central and northeastern Japan.